Time-Lapse

[Ted's]

I own a L730

Picture1:

Picture2:


This digital camera consists of a green LED next to the T on Picture2. I want the PIC to figure out whether the LED is on or off without opening the device(the camera). How can I do that - technically and in PBP?

Thanks
Ted's

[eggman]

You could use an LDR (light dependant resistor) to check whether the LED is on. Use black tape or other stuff to make sure only the light from the LED reaches the LDR. Connect the LDR between an input pin of the PIC and ground, and a say 1 MOhm resistor between the same input pin and +5V (pull up). As soon as the LED lights the resistance of the LDR will go down, pulling the pin low. You might have to experiment a bit to get the right value for the resistor.

[Ted's]

Fair enough.
How fast can a ldr react? Is a light sensitive diode/transistor suitable aswell?

[eggman]

An LDR might be a little slow, depends on the speed you need. Photo transistor might work well too, again, you'll have to experiment a bit.

[g-hoot]

Ted's,
I'm a rooky at electronics, so what the other guys are suggesting is probably better, but here's how I use a CDS cell for detecting day and night on my circuit, and I think it would work here to, as long as no outside light can get to it. The part number is from digikey: PDV-P9203-ND The specs say:

Rise Time (Typ) 70ms
Fall Time (Typ) 15ms

I don't know, but I'll bet that would work for ya.
I connect one side directly to the +5V of my board, and the other side to a pin on the PIC. This is the code that I use, and it has worked really well. What I have found using this code, is that it works with pretty much any CDS cell. I had 5 different ones, and they all detected dark at about the same light level with this code. For really good measurement of changes in light level, I'm sure there are better, and more complicated, ways to do this, but this is great for detecting day or night (or light on/off). After reading how RC time works, I'm not real sure why the value of about 46 to 49 is consistently dark on all of the cells I tested, but hey it works.
DayNight VAR BYTE
DayOrNot VAR BIT

Low PORTB.5 'Pin Connected to CDS Cell
Pause 10
RCTime PORTB.5,0,DayNight 'Read rctime value into DayNight variable

If Daynight > 46 Then 'It's dark
DayOrNot = 0
Else 'It's Daylight
DayOrNot = 1
End If

Gary

[mackrackit]

FYI g-hoot,
http://en.wikipedia.org/wiki/Photoresistor
Now Ted has code for his project too and what speed to expect.